Conventional cameras with focusable objectives are provided with an outer, axially fixed setting ring rotatably mounted on a lens barrel and provided with female threads engaging male threads of an inner guide ring which is held against rotation and is rigid with a lens mount supporting the shiftable lens or lenses, the latter usually constituting a front component of the objective. The two rings are centered on the objective axis and are therefore of a rather large diameter, resulting in considerable friction along their threaded contact surfaces. Moreover, the manual adjustment of a setting ring located forwardly of the camera body is inconvenient while automatic focusing with such an assembly is rather difficult.
In commonly owned U.S. Pat. No. 3,482,500 there has been disclosed a control mechanism for manual and automatic focusing in which a lens mount is supported by several axially extending guide rods, one of these rods having threads engaged by a nut which is integral with the shiftable lens mount. It has also been proposed to provide a shiftable lens support with an axially extending rack engaged by a rotatable worm; see German Pat. No. 1,101,801.